Migration Of Hydrothermal Fluids Research Articles

Diagenesis and mineralization in the Central African Copperbelt, implications from the reflectance of pyrobitumen and Kübler (illite crystallinity) index

The Central African Copperbelt stands out as one of the world's largest sediment-hosted Cu-Co provinces, contributing to over 60% of global Co production. A comprehensive basin analysis approach is imperative for unraveling the ore-forming processes, encompassing an understanding of the diagenesis or metamorphism that the mineralized rocks underwent. In this study, two types of pyrobitumen (burial related and burial plus hydrothermal related) reflectance values have been measured, which record maximum temperatures during deep burial and hydrothermal imprint. The calculated temperatures provide important information about the maximum burial and hydrothermal fluid temperatures in distinct regions of the Copperbelt. The average burial related pyrobitumen reflectance for Nkana, the southeastern part of the Copperbelt, ranges between 3.43 and 3.75% BRr, indicating a maximum burial temperature of about 240 °C. Moving towards the central part of the Copperbelt at Luiswishi, the average pyrobitumen reflectance varies between 2.65 and 2.87% BRr, with calculated maximum burial temperatures reaching about 220 °C. In the northwestern part (Tenke Fungurume mining district, TFMD), the maximum burial temperatures decrease to approximately 200 °C, based on the calculation of pyrobitumen reflectance at 2.10% BRr. This decreasing temperature trend from southeast to northwest corresponds to the observed decrease in metamorphic grade across the Copperbelt, ranging from amphibolite and upper greenschist facies in the Zambian part to prehnite-pumpellyite facies in the Congolese Copperbelt. The temperatures calculated during maximum burial, especially at the Nkana ore deposit, are lower than those previously proposed based on the mineralogy of the rocks, i.e. upper greenschist facies. However, the latter does correspond to the temperatures reached by the mineralizing fluids in this area and thus rather reflects the alteration assemblage and temperature. The average burial plus hydrothermal related pyrobitumen reflectance measured at TFMD ranges between 3.06 and 5.36% BRr, indicating calculated average temperatures of 300–350 °C. These temperatures align with those recorded for the late diagenetic to syn-orogenic mineralization by fluid inclusion microthermometry (180–340 °C) at TFMD. The data suggests a pervasive migration of hydrothermal fluid through the rocks, contributing to the observed pyrobitumen reflectance. The illite crystallinity of the examined samples is notably high. At Nkana, the KI values range between 0.10 Δ°2θ and 0.22 Δ°2θ, for Luiswishi between 0.12 Δ°2θ and 0.24 Δ°2θ, for Kamoto between 0.17 Δ°2θ and 0.23 Δ°2θ, for TFMD between 0.17 Δ°2θ and 0.25 Δ°2θ, which indicates all these samples were placed in epizone, with only a few in anchizone. This high value could be attributed to two potential factors: the admixing of detrital muscovite to the sediments or the influx of potassium due to the pervasive alteration throughout the Copperbelt, but the latter one is less likely since the selected samples show no evidence of alteration.

Volcano tectonic setting of the Salares Norte Au–Ag epithermal belt, central Andes of northern Chile

The Salares Norte (SN) Au–Ag belt is here defined as an Upper Miocene-Pliocene metallogenic area, extending for nearly 200 km, located NE of the northern end of the Upper Oligocene-Lower Miocene Maricunga Belt in the central Andes of northern Chile. The belt is located just east of the main reverse NS fault systems which are part of the Sierra Castillo, Potrerillos Mine and Claudio Gay Cordillera faults, limited to the north and south by the NW-striking oblique to the arc Imilac and Wheelwright-Incahuasi regional faults systems.The epithermal deposits within the SN belt are intimately associated with the occurrence of Middle Miocene volcaniclastic basins and Middle to Upper Miocene phreatomagmatic vents and their related high-sulfidation hydrothermal systems. The volcanostratigraphy of the SN epithermal belt comprises volcaniclastic sequences, subvolcanic and volcanic rocks ranging between the Lower Miocene and the Pliocene. High-angle NS reverse faults involving Paleozoic and lower Cenozoic units form the western-southwestern limit of the belt. Folded and faulted Middle Miocene volcaniclastic and volcanic sequences form the main host rock of at least 4 phreatomagmatic events that generated maar-diatreme fields and discrete maar vents, with their related subvolcanic facies, within the widespread Upper Cenozoic volcanic arc, ranging in age between ca. 15 and ca. 4 Ma. These four phreatomagmatic events are spatially and temporarily associated to 4 main high-sulfidation hydrothermal events which have altered both the phreatomagmatic rocks and the Cenozoic country rock of the explosive activity. These hydrothermal systems have formed dozens of discrete hydrothermally altered areas where argillic, advance argillic, silicification and steam-heated alteration are common. Most of the maar fields and the hydrothermal alteration systems are emplaced within a strike-slip tectonic regime with NE-trending shortening which is accommodated in the NW-striking sinistral-normal faults and NE-striking extension structures. These fault systems crosscutting interaction favor the hydrothermal fluid circulation and migration, as well as mineral precipitation on structural and/or primary lithological traps and on NE-striking tension cracks.In summary, the SN epithermal belt is an almost 200 km long and 60 km wide area, mainly NNE-oriented, where epithermal deposits have formed during the Upper Miocene and Pliocene (ca. 13-4 Ma), emplaced within Middle to Upper Miocene volcanic and volcaniclastic sequences associated to small volcanotectonic basins oriented in NS, NE-SW and NW-SE directions, in which most of the high-sulfidation systems are associated to discrete or repetitive structurally-controlled Upper Miocene-Pliocene phreatomagmatic vents and maar fields.

The Zaozigou orogenic gold-antimony deposit, West Qinling Orogen, China: Structural controls on multiple mineralization events

Orogenic gold deposits are generally thought to represent one perhaps protracted event. However, recent research on orogenic gold deposits increasingly offers evidence for some deposits forming through multiple and clearly discreet hydrothermal episodes. The giant Zaozigou orogenic Au-Sb deposit in the Triassic to Cretaceous West Qinling Orogen, central China, includes both steeply dipping and gently dipping orebodies. The two distinct mineralization styles provide a valuable setting for investigating a multiple mineralization model by integrating structural analysis within a robust geochronological framework. Through fieldwork and geochronology, we define a progression of major tectonic events in the area of the Zaozigou deposit. The deposit is hosted within a well-bedded sequence of Early Triassic metasedimentary rocks of the South Qinling Terrane. Pre-mineralization E-W shortening (D1) during subduction of the Mianlue oceanic slab include folding with resulting axial planes striking N-S, emplacement of Triassic ENE-striking and WNW-striking dacite dikes accompanied by Middle Triassic greenschist facies metamorphism. Late Triassic gold-stibnite quartz vein and disseminated mineralization formed along ENE-striking and steeply dipping D2 brittle to ductile sinistral faults. Their orientations suggest a link to the regional NNE-SSW maximum principal stress coinciding with transpression caused by the Late Triassic collision between the South China Block and South Qinling Terrane. Overprinting Early Cretaceous quartz-stibnite veins developed along gently dipping (20° to 40°) brittle D3 normal fault zones, which exhibit a NE-SW minimum principal stress. This younger deformation event is interpreted to be related to the Early Cretaceous tectonic transition from shortening to extension of the West Qinling Orogen. Therefore, the Zaozigou deposit reveals a model of multiple orogenic gold mineralizing events, with migration of hydrothermal fluids during discrete deformation episodes and the resulting formation of a single composite deposit formed along overprinting structures at separate times of orogenesis.

Structure-controlled mineralization resulted from Paleoproterozoic orogenesis in the Dahenglu Cu-Co deposit, NE China

Paleoproterozoic orogenic-related (or orogenic-like) ore deposits play a significant role in the metal and non-metal metallogenic systems in the North China Craton. This study focuses on detailed field structural of the Dahenglu Cu-Co deposit to reveal relationship between structural controls and enrichment process of cobalt (Co)-bearing hydrothermal fluids in the metasedimentary rock. In the context of Paleoproterozoic orogenesis, at least three phases of deformation were identified within the ore-bearing country rocks: an early DPl-1, the peak-orogenic deformation phase called DPl-2 and a final exhumation stage of deformation designated as DPl-3. Through meticulous structural analysis, microscopic examination of drilling cores, and the application of the TESCAN Integrated Mineral Analyzer, we have discerned that thrust faults, operating at both outcrop-scale and microscale, function as substantial conduits for the migration of ore-forming hydrothermal fluids. Additionally, fractures within thrust faults, the core of folds, and the high strain micro-scale shear zone between overturned folds provided ideal spaces for the precipitate of Co-bearing hydrothermal fluid. Based on the regional geological correlations, the second phase of deformation (DPl-2) is characterized by S2 foliation, crenulation cleavage, and thrust fault, which played a crucial role in the enrichment of cobalt in minerals such as siegenite and pyrite. This cobalt enrichment is attributed to the peak-orogenic stage, DPl-2, which likely occurred between 1.95 and 1.90 Ga. The ore-forming geodynamic was associated with Paleoproterozoic orogenesis, characterized by intense deformation and metamorphism. This conclusion aligns with cobalt enrichment observed in the Paleoproterozoic Trans North China orogen in the central North China Craton and the Paleoproterozoic Peräpohja belt in northern Finland. The structures generated by significant tectonic events, such as Paleoproterozoic orogenesis, represent key controlling factors of Co-rich ore deposits.

Superimposed multi-stage mineralization in the Qingchengzi Pb-Zn ore district, North China: Evidence from geochronology and sulfide geochemistry

The Qingchengzi Pb-Zn polymetallic ore district in northern China records two stages of Pb-Zn mineralization in the Paleoproterozoic and Mesozoic, respectively. The Paleoproterozoic sedimentary-metamorphic stage (stage I) of mineralization was primarily recorded by stratiform ores, and the Mesozoic magmatic-hydrothermal stage (stage II) of mineralization mostly formed vein-type ores. The apatites coexisting with the stage I sulfides yield U-Pb isotopic ages of (1882–1857 Ma), corresponding to the regional metamorphism pervaded in the Paleoproterozoic in the Liaodong region. Zircons from a granite porphyry that is spatially associated with the stage-II mineralization have a U-Pb age of 228.5 ± 5.8 Ma.Stage-I fine-grained sulfides (Py1a and Sp1a) are of SEDEX origin and the fluid is fertile in metals such as Co, Ni, Zn, As, Mn and V. The metals are likely to be sourced from the Liaohe Group, which contains high concentration of those metals. Stage-I recrystallized sulfides (Py1b and Sp1b) show trace elements such as Co, Ni, Cu, As and Au of pyrite and the Cu, Cd, In, Ag, Sb of sphalerite were remobilized out from their crystal lattice under metamorphism. Stage-II sulfides (Py2 and Sp2) are more enriched in Au, As, Cu, Ag, Sb, Pb compared to stage-I sulfides. LA-ICP-MS mapping reveals multigenerational texture of Py2 which features a Cu and Zn enriched core (Py2a) and an overgrowth rim (Py2b) of enrichment in Au, As, Co and Ni. The compositionally distinct core and rim presumably suggest that two episodes of magmatic fluids are involved in the formation of stage-II sulfides. The sedimentary pyrite from Liaohe Group and the stage-I sulfides have δ34S value in the ranges of 10.4–17.04 ‰ and 4.10–9.37 ‰, respectively, suggesting that sulfur was probably derived from seawater sulfate reduction. The sulfur isotopes of stage-II sulfides (δ34S = 4.10–9.02 ‰) reflects a mixing of magmatic sulfur with sulfur leached from meta-volcanic-sedimentary source. This process was associated with the upward migration of magmatic hydrothermal fluids and subsequent fluid-rock interaction, which leached heavy sulfur isotopes along with Cu, Zn, Co, Ni, V metals from wall rocks of mica schist from the Dashiqiao and Gaixian Formations. Integrated mineral assemblage, sulfide texture, trace element and sulfur isotope signatures and geochronological constraints reveal the primary Paleoproterozoic SEDEX mineralization was intensively overprinted by later metamorphism and Mesozoic magmatic hydrothermal events for the Pb-Zn deposits in the Qingchengzi ore district.

Mass transfer during hydrothermal bleaching alteration of red beds in the Lanping Basin, SW China: Implications for regional Cu(–Co) and Pb-Zn mineralization

The Lanping Basin in SW China hosts many hydrothermal Cu(–Co) and Pb-Zn deposits. Widespread bleaching alterations of red beds in the basin record mass transfer during the hydrothermal fluid migration and may provide useful information about metal sources for the deposits. Based on fluid inclusion study, mineral and elemental mapping of thin sections, and whole-rock major and trace element analyses, this study investigates fluid properties and mass transfer during the hydrothermal bleaching alterations at T1 and T2 two places where Jurassic red siltstones have been bleached in association with sulfides-free hydrothermal quartz(-siderite) veins. Mineral compositions and fluid inclusion microthermometry indicate that both T1 and T2 fluids are CO2-free, weakly acidic to neutral, S-poor, hydrocarbon-bearing reduced fluids, but the former is higher than the latter in the homogenization temperatures (188 – 265 ℃ versus 136 – 184 ℃) and salinities (17 – 36 wt% NaCl eq. versus < 11 wt.%NaCl eq.). These reduced fluids caused the bleaching alterations due to the loss of Fe3+ (in terms of hematite) from the red siltstones. The T1 fluid is comparable to the vein-type Cu-Co and Pb-Zn ore fluids in the Baiyangping district. Enrichment of Cu-Co and depletion of Pb-Zn in the bleached rocks relative to the protolith at T1 suggest that the two pairs of elements are less possibly derived from the same source rocks. Red beds probably provided Pb and Zn, while other rock types, such as mafic volcanic rocks, were likely the sources of Cu and Co. As such, various degrees of contributions from different source rock types during the ore fluid migration may determine the variable (Cu-Co)/(Pb-Zn) ratios observed in the vein-type Cu-Co and Pb-Zn deposits. The T2 fluid is similar to the ore fluids of some vein-type Cu deposits along the western margin of the Lanping Basin. Considering that the T2 hydrothermal veins are free of sulfides and the fluid inclusions from the vein-type Cu deposits in this region either contain CO2 or do not contain CO2, the abundance of reduced sulfur rather than CO2 determines whether the reduced fluids in this area are capable of causing Cu mineralization. Different from T1, Mg and TFe2O3 were introduced into the bleached rocks at T2 site. This possibly resulted from interaction between the migration fluids and widespread mafic volcanic rocks in this region. Enrichment degree of Cu is apparently greater than those of Co, Zn, and Pb in the bleached rocks, which is consistent with the Cu-only mineralization for the vein-type Cu deposits in this area.

Amorphous carbonaceous material in Paleoproterozoic pillow lavas (Onega Basin, NW Russia): Origin, source and migration

The amorphous carbonaceous material (CM) in mafic igneous rocks is important for deciphering the relationships between igneous and hydrocarbon systems in Earth interior. Deciphering of CM in Precambrian rocks it is a challenge because of reworking by later superimposed events but could give insights into evolution of these systems in time. We report the results of combined field and petrological studies, Raman spectroscopy and carbon isotope analysis of well-preserved ca. 2.0 Ga pillow lavas in the Onega Basin, Karelian craton, Eastern Fennoscandian Shield. In these rocks CM is present in both pillow and inter-pillow areas as poorly ordered carbon. In the MORB-like basalts of the Zaonega Formation, CM occurs in post-magmatic veinlets formed via migration of hydrothermal fluids. In the OIB-type basalts of the Suissary Formation, CM is found in amygdales, inclusions in feldspar, and in devitrified volcanic glass indicating a CM deposition via a rapid cooling of C-rich magma. Despite morphological diversity carbon isotopic composition indicates a biogenic origin of CM in both the MORB- and OIB-like basalts. The most likely source was the Paleoproterozoic oilfield in the Zaonega Formation, commonly referred to as the Shunga event.

Triggering mechanism and brittle-ductile dynamics of active faults in the south-central Saurashtra horst, Gujarat, western India: A geospatial, geological, and geophysical approach

The seismically active Saurashtra horst is located within the intraplate volcanic continental margin of western India. The region is prone to moderate and low-magnitude earthquakes within the depth range of ∼ 3 to ∼ 24 km. We observed that the earthquakes in this region are associated with seismically active brittle and ductile crustal layers. To understand the dynamics of the earthquake generation process, we applied an integrated geological and geomorphological approach, supplemented by subsurface geophysical (magnetotelluric) studies. Additionally, the active surface deformation has been measured using the PSInSAR and GLA techniques. Based on the stream offset and geomorphic landform development pattern several NW-SE and NE-SW oriented strike-slip faults have been identified. The PSI-derived displacement analysis reveals that the area is deforming at the rate of ± 5 mm/yr. Furthermore, subsurface crustal heterogeneity with increasing depth has been identified using the magnetotelluric technique, which is reflected in the form of basaltic lava flows, plutonic emplacement within the granitic basement, and the presence of semi-crystallized magmatic bodies below the brittle-ductile level. Additionally, we proposed a model to depict the plutonic emplacement within the highly fractured/faulted granitic basement and their relationship to the earthquake generation process. Our model shows that crustal heterogeneity and the migration of hydrothermal fluid from the semi-crystallized magmatic body along the active fault cause earthquake nucleation processes within the brittle and ductile layers. We concluded that the upwelling magmatic fluid above the brittle-ductile transition (BDT) acted as a lubricant for the nucleation and triggering of the earthquake along the active faults. Similarly, the fractured ductile crust is weakened by fluid migration, which causes high fluid pressure in the ductile crust thereby decreasing the confining pressure and endorsing the velocity weakening in the aseismic layer, responsible for the shear instability that causes deep crustal earthquakes. More specifically, the lithological heterogeneity at brittle and ductile regimes is an important factor for the earthquake nucleation process in this part of the Indian plate.

Estimation of the sediment thickness beneath the Southern Benue Trough in Nigeria by using gravity and borehole data

Mapping the sediment-basement morphology represents a crucial part of the geophysical exploration of oil, gas, and mineral resources. In this study, authors estimated the sediment thickness in the area covering the Southern Benue Trough in Nigeria and parts of the Cameroon Volcanic Line using a high-resolution tailored gravity model together with sediment and bedrock samples taken from 113 logged boreholes. A 3-D inversion of the residual gravity data is done by applying a recently developed time-efficient gravity inversion software to determine a 3-D depth structure of the basement relief after regional-residual gravity separation. The estimated depths of the sediment-basement based on a 3-D gravity data inversion are compared with the measured sediment thickness data from drilling profiles to optimally select the gravity inversion parameters, particularly the mean sediment depth, and density contrast. Our numerical result indicates that the sediment thickness within the Southern Benue Trough study area and parts of the Cameroon Volcanic Line vary from 0.8 to 5.5 km, with a prevailing southwest trend of increasing sediment thickness while decreasing eastward. This trend closely mimics the known geological structure of the study area and generally agrees with localized estimates of the sediment thickness from previous studies. Moreover, the Bouguer gravity map of the study area exhibits a spatial pattern that indicates an existence of a high-density material, which could have led to the shallowing of sediments over and along the Abakaliki Anticlinorium. Based on these findings, the authors argued that these magmatic structures could influence hydrothermal fluid migration and might have entrapped enough sediments suitable for the maturation of mineral resources over time.

Response of clastic reservoir to magmatic intrusion: Advances and prospects

Exploration practice shows that magmatism-affected clastic reservoirs have potential in oil and gas exploration, but it lacks systematic understanding of the effect of magmatic intrusion on the surrounding clastic rocks. This paper shows that the influence range of magmatic intrusion into clastic reservoirs is positively correlated with the size of magmatic body. The influencing factors mainly include compressive deformation, high temperature, and hydrothermal activity. Magmatic intrusion into clastic reservoirs not only produce physical fractures or dissolve pores, but also lead to mineral precipitation to deteriorate the reservoir space. The vertical and horizontal migration of hydrothermal fluids complicate the distribution of reservoir space and induce reservoir heterogeneity. We further explore: 1) magmatic-hydrothermal information in clastic reservoirs through the analysis of fracture-related veins at different locations in the surrounding reservoirs and, 2) modification effects of magmatic intrusion in clastic reservoirs constrained by geothermometers and anomalous temperature fields. With the guidance of theories and previous studies, we can carry out research using key techniques such as SILLi heat transfer modeling and carbonate U–Pb dating. We aim to develop a comprehensive quantitative model for evaluating the thermal response of clastic reservoirs to magmatic intrusion, and revealing the co-evolution of temperature, thermal convection response and porosity of reservoir modification effect.

Using association rules analysis to determine favorable mineralization sites in the Jiaojia gold belt, Jiaodong Peninsula, East China

Mesozoic orogenic gold deposits in the Jiaojia goldfield, Eastern China show the features of regular mineralization spatial distribution and have long been captivated the attention of geologists. This study integrates 3D modeling, model space analysis, and association rule analysis to extract the relationship between structural deformation and gold mineralization. The Apriori algorithm with spatial constraints is used to quantify the favorable mineralization sites in the Jiaojia gold field using different combinations of ore-controlling indicators. The first favorable mineralization site has a 44% probability of mineralization and is primarily located in segments of the fault footwall within 25 m of the fault buffer where the transition from steep to gentle occurs (∼1–24). Its uniform distribution indicates that the ore-bearing space near the main fault plane is relatively wide, and hydrothermal diffusion is relatively uniform. Site No. 2 has a 45% probability of mineralization and is located in convex segments (6–35 m) between 25 and 55 m of the fault footwall. The equidistant zonal distribution in this area indicates that secondary faults and fractures have facilitated the migration of hydrothermal fluids, resulting in the formation of extensive ore-bearing spaces. Site No. 3 sited in convex segments (∼0–6 m) distal to the fault footwall (∼55–515 m) of the fault buffer where the dip is gentle (∼30°–35°). The binary structure of differing dips formed by the intersection of steeply dipping orebodies and the slowly dipping orebodies in the central and southern parts of the Nanlü-Xinmu deposit exhibit the complex channel away from the main fault. In this study, the correlation between ore-controlling factors and mineralization in Jiaojia fold field is quantitatively expressed as favorable mineralization sites in the form of association rules, it would contribute to a deeper understanding of the underlying mechanisms governing the formation of structure-controlled orogenic gold deposits in the Jiaojia goldfield.

Pore Variation Characteristics of Altered Wall Rocks in the Huize Lead–Zinc Deposit, Yunnan, China and Their Geological Significance

The porosity and permeability of the rock surrounding lead–zinc deposits are key factors for controlling the migration and precipitation of ore-forming hydrothermal fluid. In this paper, the Huize super-large lead–zinc deposit was taken as the case study, and variations in the porosity and permeability of the wall rocks and their relationship with the orebody were analyzed by using CT scanning technology. The experimental results showed that the average pore radius of dolomite with a decreasing distance to the orebody ranged from 1.60 to 1.65 μm, increasing to 1.77~2.05 μm. The CT porosity increased from 2.76%–2.81% to 3.35%–3.99%. The average pore throat length decreased from 29.57–39.95 μm to 13.57–16.83 μm. In the research, it was found that the hydrothermal fluids rich in chemical elements changed the properties of the surrounding rocks. Temperature rise will lead to dolomitization of limestone and recrystallization of dolomite. This process led to an increase in the porosity of the wall rocks. During the formation of the orebody, the metal minerals in the hydrothermal fluid entered the pores of the rock. As a result, the pore radius and pore volume of the wall rocks were reduced, along with the pore throat radius and pore throat length. Therefore, the wall rock pores near the orebody were isolated from each other, and the permeability of the surrounding rock decreased. The variation characteristics for the porosity and permeability of the dolomite at various distances from the mine can be used to discover orebodies.

Local Earthquake Seismic Tomography Reveals the Link Between Crustal Structure and Volcanism in Tenerife (Canary Islands)

AbstractVolcanic activity on Tenerife Island is extremely diverse. Three radial rift zones are characterized by cinder cones from basaltic fissure eruptions. A triple junction in central Tenerife exhibits a complex of merged, predominantly phonolitic, stratovolcanoes. The Las Cañadas caldera and widespread ignimbrite deposits reveal high explosive potential. We investigated the crustal and upper mantle structure beneath Tenerife using local earthquake data recorded by two dense seismic networks on the island. For our tomographic inversion, we selected &gt;130,000 P‐ and S‐wave arrivals from ∼6,300 events that occurred during seismic unrests in 2004–2005 and 2017–2021. Synthetic tests confirmed that we could robustly resolve seismic velocity structures to ∼20 km depth. In the upper crust (down to ∼7 km) beneath central Tenerife, a prominent high‐velocity anomaly represents the rigid core of the volcanic complex; at greater depths, a strong low‐velocity anomaly reveals abrupt crustal thickening. Vp and Vs contour lines of 5.2 and 2.85 km/s, respectively, reveal Moho depth variation; crustal thickness beneath Las Cañadas reaches ∼17 km, whereas that beneath other parts of Tenerife is ∼10 km. An anomaly at ∼5 km beneath the caldera with low Vp, low Vs, and high Vp/Vs might be associated with a major phonolitic magma reservoir. Similar anomalies at ∼ sea level may represent shallow magma sources responsible for recent eruptions. Seismicity occurs in a columnar area of high Vp, high Vs, and low Vp/Vs, and may represent hydrothermal fluid migration through brittle media. Based on our results, we constructed a conceptual model of volcanic activity on Tenerife.

Evidence of hydrothermal fluid migration pathways in pores and microfractures based on nuclear magnetic resonance and electron microscopic analyses from the Southwest Indian Ridge

The formation of massive sulfide deposits on the seafloor is closely related to sub-seafloor hydrothermal circulation. Hydrothermal circulation depends on the connected pores and fractures. Most previous studies have focused on hydrothermal circulation at the macroscopic scale; however, the transport pathways of hydrothermal fluids have rarely been studied at the microscopic scale. This study showed that multiple measurement techniques could characterize the pores of rocks and the mineral fraction surrounding these pores. Nuclear magnetic resonance (NMR) was used to measure the porosity of fresh basalt, altered basalt, massive sulfide, and sulfide chimneys obtained from the Southwest Indian Ridge. Meanwhile, the distribution of pore radius and pore throats in these samples was obtained using the NMR transverse relaxation time distribution. The scanning electron microscopy results and polarizing light microscope images showed that the pores of altered basalts are more developed than those of fresh basalts, which is consistent with the NMR measurement results that altered basalts have a large pore size distribution. In addition, metallic minerals such as pyrite were found to be precipitated in these pores, and microfractures were observed in altered basalts. This demonstrates that the pores and microfractures in these altered basalts provide pathways for fluid transport and are enriched with metal minerals. Altered basalt is closely related to the formation of massive sulfide deposits and is an important ore-prospecting indicator. Therefore, the study of rock pore structures and components can provide a better understanding of hydrothermal circulation and contribute to the exploration of mineral resources.

Petrographic and mineral chemistry investigation of the high-grade chrysotile asbestos-bearing Zvishavane Ultramafic Complex, south central Zimbabwe

The Zvishavane Ultramafic Complex (ZUC) in the south central part of the Zimbabwe craton is comprised of coarse-grained serpentinites, metadunites and metagabbros and hosts Africa's largest reserves, and largest mine, of high-grade chrysotile asbestos. Magnesiohornblende, actinolite, plagioclase (An0.6–41.9), augite, diopside and clinozoisite constitute the mineralogy of the ZUC. Forsteritic olivine (>Fo90) was altered to form chrysotile and antigorite minerals, although some primary olivines are preserved. Contents of Al2O3 range from <1 to >2.5 wt% at TiO2 values of <0.7 wt% consistent with an island arc setting for the ZUC that originated from tholeiitic magmas. The Zvishavane Ultramafic Complex was metamorphosed at relatively high temperatures (542-779 °C) and low pressures (1.2-2kbars), consistent with contact metamorphism. Intrusion of several granitic batholiths relatively close to the ZUC likely triggered hydrothermal fluid migration which metamorphosed the ZUC. The associated asbestos deposits likely formed during hydrothermal circulation events. Zoned amphiboles, the occurrence of magnesiohornblende and actinolite, as well as cross-cutting serpentine veins are consistent with at least two stages of alteration and/or metamorphism of the ZUC. The lack of a thrust contact between the ZUC and its country rocks is consistent with the ZUC having intruded into the host Zvishavane Gneiss Complex and possibly acted as a feeder to the nearby Mberengwa greenstone belt (MGB). However, the occurrence of near end-member forsteritic olivine, the presence of zoned amphiboles, and faulting within the ZUC are all suggestive of an ophiolitic origin although forsteritic olivines also occur in intrusive layered complexes. Metamorphism of the ZUC, ascribed to intrusion of multiple batholiths and possibly the MGB, likely led to the formation of ZUC chrysotile asbestos deposits.

Effects of contact metamorphism on δ13C, δ15N, and XPS nitrogen functional forms of inertinite-rich Witbank coals (South Africa): Indications for possible alteration by hydrothermal fluids

In this study, δ13C, δ15N, and nitrogen functional forms were used to evaluate the effects of contact metamorphism on inertinite-rich Main Karoo Basin coals (South Africa). Eighteen samples were collected at 0.25 m intervals from the western and eastern sides of a ∼ 2.6 m thick dolerite dyke emplaced into the No. 2 Seam; an unaltered coal from the same seam was included as a control (Witbank Coalfield). Inertinite (semifusinite and inertodetrinite) dominated the organic composition of the samples. δ13C and δ15N for the unaltered coal were − 21.1‰ VPDB and 1.5‰ Air, respectively. For the intruded coals from the western side, δ13C ranged between −22.4 and −21.4‰ VPDB, and δ15N between 1.2 and 2.6‰ Air. The δ13C for samples from the eastern side ranged between −21.8 and −20.7‰ VPDB, and between 1.8 and 3.0‰ Air for δ15N. Differences in δ15N between the unaltered and the intruded coals were generally more significant than in δ13C, especially for samples from the eastern side. The positive shifts in δ15N may reflect reactions between organic nitrogen in coal and mobile nitrogen transported by hydrothermal fluids. The absence of a relationship between δ15N and distance from the dyke, may reflect secondary, localized contact metamorphism due to hydrothermal fluid migration within the intruded seam. The circulation of a fluid transporting 15N-rich moieties, apparently more active on the eastern side, was supported by the higher ash yields, δ15N, and Ntotal/Corganic ratios. The correlation between Ntotal/Corganic and ash yields suggests deposition of NH4+ − bearing inorganic components preferentially into the eastern side. Similar to the unaltered coal (N-5 = 47.24 at.%), N-pyrrolic was the dominant nitrogen functional form of the intruded coals, reflecting the developmental pathways for the inertinite macerals as opposed to the contact metamorphism. The low proportion of graphitic (quaternary) nitrogen, irrespective of sample distance from the dyke, suggests metamorphic temperatures lower than 900 °C.

Analysing multi-index overlay and fuzzy logic models for lode-gold prospectivity mapping in the Ahafo gold district – Southwestern Ghana

Mineral prospectivity mapping (MPM) is a multi-criteria decision-making process that identifies and prioritises potential zones for mineral exploration. Multi-index overlay and fuzzy logic models were employed in this paper to conduct GIS-based MPM of the Ahafo gold district, which is located on the northwestern margin of the Sefwi granitegreenstone belt in Ghana. Mineralisation in the area is structurally controlled with a high degree of hydrothermal alteration. Therefore, the mineral systems approach was used to translate the understanding of orogenic gold hydrothermal system into mappable exploration criteria, leading to the generation of nine evidential layers. These layers represented mappable spatial proxies for active fluid pathways (i.e. proximity to NEstriking basement faults: D2 to D3), physical traps (i.e. proximity to N- to NNE-strike sinistral fault: D4; proximity to litho-contacts; proximity to the intersection of N- to NNE- and NE-strike faults; density of N- to NNE- and NE-strike faults; litho-competency contrast), and chemical traps (i.e. proximity to Fe-rich dykes; litho-reactivity contrast; proximity to hydrothermal alteration zone), all of which are critical for hydrothermal fluid migration, focusing, and gold deposition. Weights and confidence factors were assigned to the evidential layers in the fuzzy logic approach based on experts' knowledge of how the layers are relevant to orogenic gold mineralising processes. In contrast, weights were assigned to layers in the multi-index overlay model based on prediction-area (P-A) plots and normalised densities, limiting the subjectivity of assigning weights to the layers. In order to delineate potential zones for lode-gold mineralisation in the study area, the weighted evidential layers were combined to create multi-index overlay and fuzzy prospectivity maps. The continuous prospective scores of the prospectivity maps were discretised and classified using a concentration-area (C-A) fractal model. The resulting discretised prospectivity maps based on the location of 18 known lode-gold occurrences (i.e. deposits and economic prospects) were further evaluated using P-A plots. The discretised multi-index overlay prospectivity map predicted 76% of known lode-gold occurrences, with 24% of the study area as prospective, whereas the discretised fuzzy prospectivity map predicted 74% of known lode-gold occurrences, with 26% of the study area as prospective. The high potential zones identified on the prospectivity maps were spatially located in areas with structural complexity along: (i) NE-SW structural trends (i.e. 40°–50°); and (ii) secondary structural trends in the NS to NNE-SSW direction (i.e. 0°–10° and 15°–25°). These structural trends control lode-gold mineralisation in the Ahafo gold district..

Multiple fluid flow events and diversity of hydrothermal minerals in Neoproterozoic to lower Paleozoic carbonate reservoirs, Tarim Basin, NW China

• Two stages of hydrothermal events account for carbonate-hosted mineralization . • Quartz precipitated from silica-rich fluids from the Precambrian sandstone layers. • Saddle dolomite, fluorite and calcite formed in the Cambrian shale-derived fluids. • The same parent fluid could precipitate different minerals in differing host rocks. • Basal sandstones do not always act as lateral conduits for hydrothermal fluids . Contrasting mineral assemblages have been frequently found in Neoproterozoic-lower Paleozoic carbonate reservoirs along high-angle faults in the Tarim Basin. However, the reason for the change in mineral types over time is poorly constrained. Field and petrographic observations, fluid inclusion microthermometry, rare earth element (REE) and C, O and Sr isotope analyses are used to constrain the changes in water chemistry and fluid flow during the development of the mineral assemblages. During the Middle Devonian-early Carboniferous, the flux of hydrothermal fluids from Neoproterozoic sandstone layers, linked to the activity of NE–SW-trending strike-slip faults, resulted in extensive silicification. The silica-bearing fluids were hot (128.1–209.4 °C), saline (17.1–21 wt% NaCl eq.) and enriched in radiogenic strontium and showed no link to large-scale petroleum migration. During the Permian, a regional thermal anomaly induced the expulsion of hydrocarbon-bearing hydrothermal fluids from lower Cambrian shales, where Mg 2+ , 87 Sr and F − were derived via the transformation and leaching of clay minerals. A more significant in situ source of Mg 2+ was the dolostone interval, as the migrating fluids contributed to the precipitation of saddle dolomite with high T h values (82–176.4 °C) and salinities (15–21.4 wt% NaCl eq.), negative δ 18 O values and radiogenic Sr compositions. The succession of saddle dolomite followed by fluorite with similar T h values, salinities and 87 Sr/ 86 Sr ratios indicates a Mg 2+ deficit and increased dissolved calcium activity in the same parent fluid system, which facilitated the breakdown of MgF − and combination of Ca 2+ and F − . Subsequently, cooling and desalination of the hydrothermal fluids occurred, which was induced by the infiltration and mixing of meteoric water related to the compression and uplift of the north-western flank of the Tarim block. These processes resulted in the precipitation of milky white calcite (Cal 1) with similar oxygen and strontium isotope compositions but wider ranges of T h (82.1–154 °C) and salinity (3.4–16.9 wt% NaCl eq.) than those of the saddle dolomite and fluorite. The lateral flow of hydrothermal fluids through suitable layers, such as bed interfaces, permeable aquifers and karstic units, facilitated the formation of stratabound mineral deposits away from the feeding faults. The termination of hydrothermal mineralization was marked by the precipitation of brown calcite (Cal 2), whose meteoric origin has been proven by the combination of its reddish staining, oscillatory luminescent zonation and low δ 18 O values. This integrated study approach improves the understanding of the diversity of exotic mineral assemblages in carbonate hosts and the evolution and migration of hydrothermal fluids connected to late Paleozoic tectonism.

Lisbon Field: Geology of microseepage

Certain satellite sensors can be used for mineral identification and thus detection of alteration indicative of hydrocarbon microseepage. However, care must be taken when placing these results into context because alteration also can be due to other factors. The authors review the rationale for multiscale and multispectral image selection as well as the benefit of iterative processing, to highlight alteration within a given formation. The authors determine using remote sensing results integrated with other geologic information to confirm that alteration exists, compare results with a conceptual microseepage model, reconcile alteration pattern and extent with the geologic setting and trap timing, and appraise the potential that other factors may have caused alteration. The authors used geologic information from the well-studied Lisbon Field including, but not limited to, geochemistry, petrography and diagenetic studies, and paleo-structure reconstruction. We evaluated whether the alteration model made sense by assessing the thermal history and estimating volumetrics and seepage rate based on the geologic setting and extent of alteration. The results confirm that the alteration at the Lisbon Field is compatible with hydrocarbon microseepage as its principal cause, however, it cannot rule out the possibility that some of it may be due to hydrothermal fluid migration.

The Effect of Different Outer Cations on the Stability of Fluorotitanium Complex

Fluoride-rich fluid is believed to be able to activate and migrate Ti and other high field-strength elements to the greatest extent. The stability of F-rich titanium complexes can ensure their migration in the fluid, but is inseparable from the physical and chemical properties of the fluid, such as concentration, temperature and pH value—important factors affecting the stability of the complexes. In this study, the influence of the outer cationic complex fluid on the stability of the fluorine titanium complex was studied. Studies were based on different kinds of fluorine titanium complex (K2TiF6, Na2TiF6, (NH4)2TiF6 and H2TiF6) in 100 MPa pressure. Under the condition of 200~500 °C temperature, we found that as the temperature rises, the hydrolysis of F-rich titanium complexes is violent. We compared the stability of four F-titanium complexes with different outer cations according to the hydrolysis rate and the cumulative hydrolysis equilibrium constant. We compared the F-titanium complexes with alkali metal as the outer cations that are more stable, such as K2TiF6 and Na2TiF6. However, the F-rich titanium complex in an acidic fluid is relatively unstable, which is not conducive to the migration of Ti elements. Due to the water–rock reactions that occur in hydrothermal fluid migration, mixing and alteration, once in the hydrothermal system, the fluid composition, pH value and temperature change. Thus, the F-titanium complex becomes extremely unstable, leading to the precipitation of titanium from the hydrothermal fluid and the growth of Ti-rich minerals.